Dark trace storage tube



May 27, 1969 P. sl-:ATs

DARK TRACE STORAGE TUBE Filed April 27. 1965 ...wif

INVENTOR PETER SEATS A927@ @5g/L ATTORNEYQSL United States Patent O US.Cl. 315-10 10 Claims ABSTRACT F THE DISCLOSURE The present inventionrelates to a dark trace storage cathode ray tube and novel screencombination utilizing radiant energy reflecting means for erasure of thescreen without reducing the useful life or uniformity of operation ofthe tube. This dark trace storage tube and novel screen is superior toother prior art tube assemblies and is provided with a novel screenincorporating a simplified black body technique. The scotophoric layerof this invention is not subject to deterioration nor flaking, has longerasure life and in addition the tube asembly is provided with novelmeans for reliecting heat or radiant energy to impinge upon the screenso as to erase the opacity centers from the screen.

This invention relates to electron trace tubes and more particularly toso-called dark trace tubes employing a cathode ray responsive screen.

As is well known, dark trace tubes employ a screen consisting of a layerof crystals which exhibit tene- -brescence or formation of dark colorcenters when bombarded by relatively high velocity electrons from anelec- Itron gun. Examples of such crystal screens are those of thealkali halide type formed of crystals of an alkali halide or mixture ofsuch halides, for example potassium iodide, potassium chloride and thelike.

Once an opacity center is formed on the screen of a dark trace tube ittends to remain dark or opaque until measures are taken to restore theindividual darkened areas to their normal light conduction or lightreilection condition. Two methods are normally used to effect theerasure of the drak trace record. One such method uses an electron beamto effect the erasure.

A customary way of effecting erasure by an electron beam is to use thesame electron gun that is used for the dark trace recording, and togreatly increase the beam current for erasure as compared with the beamcurrent for recording. Due to the extraordinary high beam currentnecessary for erasure the life and uniformity of the gun are severelycurtailed with this method. In order to lengthen the life of the tube asecond gun similar to the recording or writing gun but which comes intooperation only during erasure is sometimes used when the electronerasure method is employed. But the use of a separate erasure gun alsointroduces a number of disadavantages. First the additional space neededfor location of the separate gun is as a practical matter not availablein the customary cathode ray tube construction and a separate tube neckmust be used to house the separate gun. This requires separate deectionand beam focussing coils on the two necks with the likelihood ofundesirable mutual interaction. Secondly the provision of two separatenecks complicates the manufacture and assembly and greatly increases thecost of the finished tube. The tube becomes unwieldy to handle andrequires extensive change in the mechanical and electrical design of theassociated equipment with which it is to 'work. Perhaps the chiefobjection to the employment of an electron beam to effect erasure is theneed of providing a high wattage high 3,447,020, Patented May 27, 1969voltage power supply in order to provide the high current necessary.

The second method to effect the erasure of the dark trace record, andthe method used in the present invention, employs heat adjacent the darktrace screen to restore the screen to its normal condition. While thismethod does avoid the disadvantages inherent in the previous electronerasure systems, until the discovery of the present invention thismethod had not proven entirely satsifactory. In previous systemsutilizing heat to restore the dark trace screen to its normal conditionthe screen has been made up of a sheet of mica or glass which carriesthe coating of a tenebrescent material such as an alkali halide, thisalkali halide layer has been covered with a coating of aluminum ofcritically controlled thinness for absorbing energy for erasing the darktrace of the tenebrescent layer. These previous dark trace tubes weresubject to early switching life failure related to mechanical andelectrical instability of the alkali halide screen and associated layersresulting in progressive physical screen deterioration includingdiscoloration and flaking. It was found that these failures resultedfrom the fact that while thin aluminum coatings deposited on glass ormica are normally ductile enough to resist shear when the substrateexpands uniformly, the alkali halide layer appears to shear transversely(as well as possibly laterally) after about two thousand erasure cycles,to assume a -fne criss cross or crazed texture. The aluminum layerdeposited on the alkali halide is correspondingly sheared as the localalkali halide fissures appear and consequently, as islands form in thealuminum coating, electrical discontinuity results. Thus, when thescreen is scanned with an electron beam, very high field gradients willappear across the fissures and current ow will be accompanied by arcingover, producing destructive effects. As the erosion of the aluminumlayer continues, severe-nonuniform screen charging arises causingdefocusing, discoloration, tracking and other effect-s.

An effective erasure system for dark trace storage tubes employing heatmust provide for not only a fast decay or erasure rate but must rapidlydissipate the heat utilized to effect the erasure dissipated in order toallow a new trace to appear on the screen. While it is relatively easyto raise the temperature sufficiently to erase the screen, previouslydiiiculty has often arose in dissipating this heat rapidly for theformation of a new trace.

It is the primary object of the present invention to provide an improvedarrangement for enabling the record on the screen of a dark trace tubeto be erased without reducing the useful life or uniformity of operationof the tube and Without greatly increasing the cost of manufacture ashas been the case with previous tubes of this type.

Another object is to provide an improved construction of dark tracetubes wherein the erasure can be eifected with rapidity and wherein theerasing action does not introduce any undesired coloring elfect on thescreen.

It is another object of the present invention to provide an improveddark trace storage tube wherein the erasing action does not producedeterioration and fiaking of the screen.

It is another object of the present invention to provide an improveddark trace storage tube having a screen including a thin sheet of mica,a layer of alkali halide crystals and a black body light absorbing layerfor erasing the alkali halide crystal layer, with the black body layerpositioned on the screen facing the cathode ray tube faceplate and thealkali metal halide layer positioned on the beam side of the screen.

It is another object of the present invention to provide a dark tracestorage cathode ray tube including means to illuminate the screen fromthe rear so as to increase contrast of the screen.

It is another object of the present invention to provide a dark tracestorage cathode ray tube in which the inner surface of the funnel of theCRT possesses an aluminum coating for focusing and reecting radiantenergy forward to the screen.

It is another object of the present invention to provide an improveddark trace storage tube having a screen including a thin sheet of mica,a layer of alkali halide crystals and a black body light absorbing layerfor erasing the alkali halide crystal layer with the black body layerlpositioned on the screen facing the cathode ray tube faceplate and thealkali metal halide layer positioned on the beam side of the screen, anda tungsten filament for providing the source of energy necessary forerasing the screen positioned about the periphery of the screen so as toavoid casting any shadow on the screen.

It is another object of the present invention to provide an erasuresystem for a dark trace storage tube providing a fast decay rate andrapid dissipation of heat from the screen so that a new trace may beformed.

Other objects and advantages of the present invention will becomeapparent from the following description taken in conjunction with thedrawings which accompany and form part ofthe specification.

In the drawings:

FIGURE l is a plan view partly in section, of a typical dark trace tubeembodying the invention.

FIGURE 2 is a magnified sectional view of the screen of FIGURE 1.

FIGURES 3 and 4 represent respective modifications of the screen ofFIGURES 1 and 2.

Briefly the present invention pertains to a dark trace storage cathoderay tube in which a conventional electron gun and control means areemployed for bombarding a screen with electrons and heat is used toerase the screen. Included in this tube is a source of radiant energy, adark trace screen consisting of a thin sheet of mica or glass, a layerof tenebrescent material such as an alkali halide facing the electrongun which darkens under electron bombardment and a black body lightabsorbing layer on the side of the screen facing the CRT faceplate toeffect with radiant energy emitted from the radiant energy source asuicient temperature rise directly within the mica to cause erasure ofthe layer of tenebrescent material, a coating of aluminum on the aredportion of the cathode ray tube for reflecting and focusing the radiantenergy forward from the source to the screen and means for providingrear lighting for improving the contrast on the screen including meansfor varying said contrast.

Referring to FIGURE 1, there is indicated by the numeral any well-knownform of cathode ray tube envelope, comprising the elongated neck portion12 joined to the flared portion 14, which is closed by the transparentend wall 16. Attached to the neck 12 is any well-known plug in base 18carrying the usual contact prongs 20. The various lead-in wires andelectrode support wires (not shown) are sealed through the stem orheader (not shown) which closes otf the neck 12 and through which thetube can be evacuated in known manner. Mounted within the neck is anywell known form of electron beam forming means comprising for examplethe electron emitting cathode 22, control grid 24, first acceleratinganode 26, and second accelerating anode 28. These electrodes forsimplicity of showing are indicated schematically in FIGURE 1, sincetheir actual structure is wellknown. Suflice it to say that by suitableheating of the cathode, and by suitable energization of the anodes bydirect current voltage, the electrons from the cathode are formed into abeam which can be focused upon the recording screen by the conventionalfocusing yoke 30. Surrounding the neck are the usual magnetic yokes 32,34, for subjecting the focused beam to the desired pont-by-pointscanning motion over the screen.

Mounted at the end of the tube adjacent the end wall 16, in the darktrace recording screen and erasure assembly according to the invention.The screen proper, comprises a transparent sheet or support 35 of micaor thin glass which is coated on the side facing the electron gun with alayer 36 of tenebrescent material such as potassium iodide crystals,`potassium chloride crystals, or crystals of other tenebrescent halideor halides. On the side of the tenebrescent layer 36 away from theelectron gun and facing the faceplate of the cathode ray tube a blackbody light absorbing layer 38 is provided such as chromium oxide orNichrome. While the black body 38 is preferably applied to the oppositeside of the mica sheet from that of the tenebrescent layer 36, it mayalso be positioned between the tenebrescent layer and the mica sheet orthe black body light-absorbing material may be intermixed with the mica.However, it is important that the black body be positioned between thetenebrescent layer and the faceplate of the cathode ray tube rather thanbetween the tenebrescent layer and the electron gun for if the blacklayer were positioned between the tenebrescent layer and the electrongun it would interfere with the electron beam and the resolution of thedark trace in the tenebrescent layer. The use of mica in the screenstructure provides for rapid dissipation of heat from the screen andconsequently a fast cooling rate.

Mounted in front of the screen on the side facing the electron gun 12 isa source of radiant energy 40 for uniformly heating the entire surfaceof the black body lightabsorbing layer 38 by radiant energy so as toprovide a sufficient temperature rise directly within the mica to causeerasure of the layer of tenebrescent material 36. In accordance with theinvention the source 40 is a fine wire tungsten filament which can beconnected to a suitable current supply source (not shown) to heat thelilament to a temperature at which it acts as an efficient source ofradiant energy. In order that the erasure filament 40 shall not cast ashadow on the screen or otherwise interfere with the recording, it maybe in the form of a very fine wire annular or toroidal Winding which ispositioned around the periphery of the screen and is outside the maximumdeection angle of the beam 42.

An aluminum coating 44 on the inner wall of the flared portion of thecathode ray tube focuses and reflects the radiant energy forward to thescreen.

Since the optimum mica dark coating or black body layer results inapproximately a 25% transmittance the screen would appear dark with onlyconventional front lighting.

In the present invention rear lighting 46 facilitated by a clear Window48 in the cathode ray tube coating has the effect of furnishing thenecessary brightness with a dark trace ratio equal to or somewhatgreater than that formerly attained by previous dark trace storagetubes. Since cast on the screen has the tendency to bleach the traceaway, the use of a light which emits excess heat must be avoided. Acircular green phosphor fluorescent lamp for rear lighting is thereforeutilized providing cool uniform lighting with minimal heating andexcellent contrast on the screen.

Provision for varying the intensity or me rear lighting 46 and as aresult the contrast of the dark trace tube is provided by varying thearea of the window 48 into the CRT by masking or mechanical shuttermeans.

Thus, the present invention provides a new improved dark trace storagetube which while employing heat adjacent the dark trace screen as anerasure means is not subject to the early switching life failureprevalent in previous dark trace tubes of this type.

The thin layer of aluminum which was subject to deterioration andflaking has been eliminated from the screen and the new screen structurein combination with the rear lighting means not only provides longerswitching life but affords improved means for adjusting the screencontrast as well as greater efficiency by decreasing the erasure time.

While the preferred embodiment of the invention is illustrated it isobvious that various changes and modifications may be made thereinwithout departing from the spirit and scope of the invention.

What is claimed is:

1. Cathode ray tube apparatus comprising in combination, an enclosingevacuated envelope, means to develop a beam of electrons, a screen uponwhich said beam impinges to make a record, said screen including atenebrescent layer of material facing said beam which develops opacitycenters when said beam impinges thereon, and means to erase saidcenters, the last mentioned means including a heatable filament withinsaid envelope for emitting radiant energy, a reflective coating on theflared portion of said tube for reflecting and focusing said radiantenergy to said screen and a black body light absorbing layer of materialforming part of said screen for providing suflicient heat to eraseopacity centers in the tenebrescent layer, said screen beingsufficiently transparent to permit the passage of visible light.

2. A cathode ray tube apparatus comprising in combination, an enclosingevacuated envelope, means to develop a beam of electrons, a screen uponwhich said beam impinges to make a record, said screen including atenebrescent layer facing said beam which develops opacity centers whensaid beam impinges thereon, a thin sheet of mica supporting saidtenebrescent layer, rear lighting means positioned about the flaredportion of said cathode tube so as to increase the contrast of saidscreen and means to erase said opacity centers on said screen, the lastmentioned means including a heatable filament Within said envelope foremitting radiant energy, a coating of aluminum on the flared portion ofsaid tube for reflecting and focusing said radiant energy to said screenand a black body light-absorbing layer of material forming part of saidscreen positioned between said tenebrescent layer and the faceplate ofsaid cathode ray tube for providing suflicient heat to erase the opacitycenters in the tenebrescent layer.

3. A cathode ray tube apparatus as defined in claim 2 wherein saidheatable filament is made of tungsten and is positioned about theperiphery of the dark trace screen.

4. A cathode ray tube apparatus as defined in claim 3 wherein said rearlighting means includes a circular green phosphor fluorescent tube.

5. Cathode ray tube apparatus comprising in combination, an enclosingevacuated envelope, means to develop a beam of electrons, a screen uponwhich said beam impinges to make a record, said screen including atenebrescent layer of material facing said beam which develops opacitycenters when said beam impinges thereon, rear lighting means to improvethe contrast of said screen including a circular green phosphorfluorescent tube located about the periphery of the flared portion ofsaid tube, a circular window in the flared portion of said cathode raytube having a `variable area for controlling the amount of rear light tothe screen and means to erase said opacity centers on said screen, thelast mentioned means including a heatable filament Within said envelopefor emitting radiant energy, a coating of aluminum on the flared portionof said tube for reflecting and focusing said radiant energy to saidscreen and a black body light-absorbing layer of material forming partof said screen positioned between said tenebrescent layer and thefaceplate of said cathode ray tube for developing sufficient heat toerase the opacity centers in the tenebrescent layer.

6. Cathode ray tube apparatus as defined in claim 5 wherein said blackbody layer is made of chromium.

7. Cathode ray tube apparatus as defined in claim 5 wherein said blackbody layer is made of Nichrome.

8. In a dark trace storage cathode ray tube of the type using =heat toerase the screen employing an electron gun and control means forbombarding a screen with electrons, a source of radiant energy, a darktrace screen consisting of a thin sheet of mica which is transparent tovisible light, a layer of tenebrescent material which darkens underelectron bombardment on the surface of said sheet facing said gun and ablack body energy-absorbing layer to effect as a result of the radiantenergy emitted from the radiant energy source a suicient temperaturerise directly within the mica to cause erasure of the layer of thetenebrescent material, a reflective coating on the flared portion of thecathode ray tube for reflecting and focusing the radiant energy forwardfrom the source to the screen and means for providing rear lighting forimproving the contrast on the screen including means for varying saidcontrast.

9. A cathode ray tube apparatus comprising in combination an enclosedevacuated envelope, means to develop a beam of electrons, a screen uponwhich said beam i-mpinges to make a record, said screen including atenebrescent layer facing said beam which develops opacity centers whensaid beam impinges thereon, a thin sheet of mica supporting saidtenebrescent layer, a circular green phosphor fluorescent tubepositioned about a flared portion of said tube so as to increase thecontrast of said screen and means to erase said opacity centers on saidscreen, the last mentioned means including a heatable tungsten filamentpositioned about the periphery of the screen for emitting radiantenergy, a coating of aluminum on the flared portion of said tube forreflecting and focusing said radiant energy onto said screen, includinga Window provided in the aluminum coating on the flared portion of saidtube and mechanical means to vary the area of the window so as to varythe amount of rear lighting and a black body light-absorbing materialforming part of said screen positioned between said tenebrescent layerand the face plate of said cathode ray tube for providing suflicientheat to erase the opacity centers in the tenebrescent layer.

10. A cathode ray tube screen for a dark trace storage tube consistingessentially of a thin supporting sheet incorporated with a black bodymaterial said sheet being transparent to visible light and saidtransparent sheet coated on the exterior and exposed surface which facesthe electron gun of the tube with a tenebrescent layer of material thatis capable of forming opacity centers thereon when bombarded withcathode rays.

References Cited UNITED STATES PATENTS 2,755,404 7/1956 Levy 313-912,836,753 5/1958 Hodowanec 313-91 2,836,754 5/1958 Holborn et al 313-913,148,281 9/1964 Fyler S15- 10X 3,253,497 5/1966 Dreyer 313--91 X3,277,241 10/1966 Spencer 178-7.87 X

OTHER REFERENCES IBM Technical Disclosure Bulletin, R. K. Mason, DarkTrace Tube Screen, vol. 7, No. 9, February 1965, p. 836.

RODNEY D. BENNETT, JR., Primary Examiner. JEFFREY P. MORRIS, AssistantExaminer.

U.S. Cl. X.R. 313-91

